The diseases and arthropods that attack blueberries can inflict severe production losses to growers by lowering yield, reducing quality, and shortening the life of a stand. Although nearly 200 diseases and more than 300 insect species have been reported on blueberries, only 20 to 25 ever become abundant enough to cause economic losses, and only 5 or 6 of these are chronic problems that require control every year. As a rule, the most damaging pests are those which attack the buds, destroy the fruit, or threaten survival of the plant.
Maintaining a clean, healthy stand of blueberry bushes is one of the most effective ways to insure good control of diseases and arthropod pests. Weedy fields and field margins provide shelter and overwintering sites for some pests, and dead canes or dying bushes can serve as breeding grounds or sources of inoculum for others. Regular cultivation of the soil kills insects that pupate under dead leaves or near the soil surface, and a good program of fertilization and water management keeps plants vigorous and better able to tolerate small amounts of injury.
In addition to clean cultural practices, a successful blueberry grower also needs a carefully planned strategy for applying pesticides. The effectiveness of control operations is influenced not only by the types of chemicals used, but also by the method and timing of their application.
In general, there are eight time periods throughout the year when it may be appropriate to use pesticides to suppress diseases or arthropods on blueberries. The following synopsis describes which pests can be controlled during each period, how to estimate optimal timing for treatments, and what other management options should be considered. Although a different spectrum of pests can be controlled during each interval, growers do have a little flexibility in the management of some minor pests (such as scales). A few pests may require multiple applications, but individual fields should rarely need treatment during more then three or four time periods during the year. This summary is no substitute for experience and careful observation. However, it does outline some of the important considerations that should go into making sound pest management decisions.
- DORMANT SEASON
The fall or winter (after leaves have fallen) is the best time to look for STEM CANKER lesions. This disease begins early in the spring when spores penetrate open stomata and produce red lesions on young, actively growing shoots. These lesions enlarge gradually over several years until the stem becomes girdled and dies. At least eight races of stem canker are present in the eastern United States. No remedial or preventive treatments are effective against this fungus, but the disease has been successfully managed in North Carolina by planting cultivars that are resistant to the dominant biotype (race 4).
Dormant oil can be applied to blueberry bushes on warm days (>50°F) in late winter before flower buds begin to open. The oil is an effective way to kill overwintering populations of TERRAPIN SCALE, wax scales, or other scale insects that infest blueberry plants. Use only a high grade petroleum oil (70-second superior type) labelled and sold specifically as an insecticide. Mix 2-3 gallons of oil in 100 gallons of water and apply 50 to 100 gallons of water per acre (depending on bush size). Thorough spray coverage is essential for good scale control.
TWIG BLIGHT is a fungal disease that spreads in early spring by spores released from dead twigs (infected the previous season). Infected terminals quickly die back 2 to 6 inches and destroy fruiting potential for an average of 6 buds per twig. Twig blight can be controlled with two or three applications of Benlate. Sprays should be applied at 7-10 day intervals from bud swell through full bloom.
MUMMY BERRY is a fungus disease that overwinters in the soil where infected, mummified fruit have fallen the previous year. Spores infect new leaves just as they begin to unroll from the bud. Mummy berry inoculum can be reduced by collecting and destroying infested fruit, or by discing the soil to bury mummies from the previous season (those covered by more than two inches of soil seldom germinate). The disease can be suppressed chemically by using Funginex (triforine) every 7-10 days between bud break and full bloom. Benlate will control secondary (fruit) infections, but it is ineffective against primary (leaf) infections.
Insect control is rarely needed before petal-fall, but occasionally fruit buds must be protected from outbreaks of CUTWORMS, SPANWORMS or CRANBERRY WEEVILS. These insects can usually be controlled with a single pre-bloom spray applied just as fruit buds begin to swell. Use an insecticide (such as Guthion) that is effective at low temperatures and avoid micro-encapsulated formulations that bees might mistake for pollen grains.
Because of the importance of honey bees and bumble bees in the POLLINATION of blueberries, no insect control activities are advisable during bloom.
Effective management of BLUEBERRY STUNT DISEASE requires season-long leafhopper control combined with prompt removal of infected plants. In the North, one spring application of insecticide has given satisfactory leafhopper control, but in the South additional sprays are needed later in the season to prevent spread of the disease by migrants of the second and third generations. Yellow sticky traps placed on low vegetation in the woods are a good way to monitor leafhopper populations. Record trap catch weekly and apply a cover spray to fields and field margins when the trap catch of adults begins to increase for each generation.
BLUEBERRY MAGGOT is the most important insect pest to look for just before harvest. Adult flies can be detected before they reach damaging levels by trapping them on yellow sticky boards (2-4 traps per acre) baited with ammonium acetate or protein hydrolysates. A treatment threshold of three adults per trap per week (or five adults per field per week) gives adequate lead time for chemical control if the traps are in place before the first flies emerge.
When control is necessary, a short-residual pesticide (e.g. Malathion) should be used. Apply this spray from the ground whenever possible to get maximum coverage on the lower half of the bushes. If ripe berries are already present, the available crop should be harvested just before the field is treated. If weather or labor conditions prevent harvest and ground application within five days, affected fields should be sprayed by air immediately. Insecticide applications should be repeated every 7-10 days until all unharvested fruit has dropped. If possible, use a picking machine to strip remaining fruit; this eliminates possible oviposition sites and should reduce future populations. Following this strategy for several years should allow growers to gradually eradicate spot infestations of the blueberry maggot.
FIRE ANTS can be both a nuisance and a health hazard to people who work in blueberry fields. Nest sites (mounds) can be treated by drenching them with a dilute solution of diazinon. Dissolve one pound of the 50 WP formulation or dilute one pint of the AG500 formulation in 100 gallons of water and slowly pour one to two gallons of this mixture onto each mound (use about 1 gallon per 6 inches of mound diameter). The insecticide works on contact to kill the worker ants, larvae, and (hopefully) the queen as well. If all the ants are not killed, survivors may construct small satellite nests nearby. A follow-up treatment may be needed a few days later to kill these mounds. All drench treatments must be completed at least 18 days before harvest.
Several species of fungi cause LEAF SPOTS that develop in mid-summer. Light infestations are generally inconsequential, but severe ones can cause premature defoliation, weaken the plant, and reduce fruiting potential for the following year.
Fungicidal control of leaf diseases is best accomplished early in the season by spraying susceptible cultivars BEFORE large numbers of leaf spots appear. A pre-harvest combination spray of Captan+Benlate applied in early May will significantly reduce the amount of leaf disease that appears after harvest. Care must be taken not to apply Benlate within 21 days of harvest.
A foliar application of Sevin during harvest will give temporary relief from FIRE ANTS that were not killed by mound drenches earlier in the season. One treatment (0.5 lb. a.i. per acre) on the day before picking should suppress foraging workers for several days, but it will not kill the queen or larvae inside the nest.
FRUIT ROT FUNGI can cause severe losses during and after harvest if berries are not handled and stored properly. Timely harvesting, sanitation, and post-harvest cooling are essential for maintaining fruit quality because fungicides applied at harvest (or in post-harvest storage) are NOT effective.
A small mound of yellow sawdust (frass) on the ground near the base of a blueberry plant is good evidence of infestation by a STEM BORER. The larva of this beetle tunnels inside the cane and pushes its sawdust-like excrement out through a small hole. Infested canes are easy to spot by looking for mounds of frass. This pest is seldom very abundant and it can be controlled simply by pruning out affected canes.
Post-harvest disease management involves continuation of biweekly leafspot sprays and scouting for problem areas. Two types of disease generally cause premature defoliation: leaf spots and root rots.
Defoliation of a bush by LEAF SPOT DISEASES occurs from the bottom up, with the oldest leaves falling off first. Once a bush is severely defoliated, fungicidal sprays are not effective. However, growers should record problem areas and be ready to apply pre-harvest and early summer leaf spot sprays the following season.
Leaf drop caused by root diseases, primarily PHYTOPHTHORA ROOT ROT, can be distinguished from defoliation caused by leaf spot fungi because root rot problems usually occur in discrete areas where drainage problems exist. Look for defoliated bushes associated with areas of excessively wet soil. Bushes suffering from root rot usually have smaller leaves that begin turning red or yellow in mid to late summer. Improved drainage is the only long-term solution for root rot problems.